Reliability at Sea of Gimbal-suspended gravity meters with 0.7 critically damped accelerometers

Abstract

Since the spring of 1962, various reliability determinations have been made at sea with gimbal-suspended LaCoste and Romberg gravity meter S-9 with its accelerometers approximately 0.7 critically damped. Measurements used for determining the meter reliability were obtained in special tests past an offshore platform where gravity was known, over a gravity range, from comparisons with previous pendulum measurements obtained in a submarine, and from intersections of gravity lines obtained on different surveys. The measurements were made in the Gulf of Mexico and in the Pacific Ocean on four different ships, ranging in size from 150 to 3000 tons. Measurements were found to be reliable when the meter beam recordings were straight and continuous, and unreliable when the beam recordings were nonlinear and oscillatory. When both horizontal and vertical accelerations are fairly uniform or nearly sinusoidal, beam recordings are linear and the measurements are reliable. When either the horizontal or vertical accelerations are impulsive or nonsinusoidal, beam recordings are oscillatory and measurements are in error, usually by +15 or more milligals. The controlling factor in reliability of measurements is magnitude and uniformity of the accelerations, not ship size (within broad limits) or straightness of steering. Exclusive of navigation errors, measurements can be accurate to within 3 mgal at Browne corrections up to 300 mgal, within 5 mgal at corrections of 300 to 400 mgal, and within 8 mgal at corrections of 400 to 500 mgal. As Browne corrections increase, the errors in meter S-9 become systematically more positive.